With respect to computers, almost everyone is familiar with the PC-type computer having a keyboard and monitor. However, there are many applications where a plurality of computers are connected to perform different functions, such as in a control room of a large building wherein one computer operates the elevators, another computer operates the heating system, and perhaps a third computer operates the lighting system.
Another use occurs in the networking of computers wherein separate computers function as file servers that provide information to remote users. In these applications, the computers may be automatically powered up or down as needed and may be infrequently accessed on location for maintenance or to obtain statistical data therefrom. Computers utilized in this mode typically are not connected to a keyboard and monitor, this being done only for the purposes described, i.e., for maintenance or to obtain statistics.
To this end, the applicant, at Cybex Corporation in Huntsville, Ala., has been manufacturing and selling a device known as COMMANDER since Dec. 15, 1989. The COMMANDER modules sold after this date contain circuitry that selectively connects one of a plurality of computers to a single keyboard and monitor, and which incorporates the circuitry disclosed herein.
Problems with using PC-type computers without a keyboard and monitor are that in the process of powering up, or "booting" the computer, such as after a power failure, and particularly with computers having internal BIOS programs that poll inputs of the computer to determine what peripheral devices are coupled thereto, a relatively complicated procedure of handshaking, parity checks, etc., occurs between the computer and keyboard. In many cases, if the computer does not detect a keyboard during this "handshaking" process, the computer indicates a keyboard failure and goes into a "wait" state.
At this point, many computers require an F1 keystroke before continuing the boot process. This is equivalent to a scan code of "3B" hex for PC/XT-type computers and "05" hex for PC/AT-type computers. This data is transmitted using a clock/data two-wire serial transmission system. This keystroke is used to manually indicate that a keyboard is now connected, bypassing the handshaking that occurs between keyboard and computer and allows the computer to complete the boot process. Applicant believes keyboard emulators that emulate an entire keyboard have been manufactured by others and which uses microprocessor-based circuitry and a relatively large amount of associated logic to implement the complicated handshaking process in order to "boot" a computer without a keyboard. However, these devices have not been successful in the aforementioned market of users of multiple computers that are not each connected to keyboards. This failure of others may be due to the fact that much of the circuitry and associated cost of a keyboard emulator is unnecessary, as in most cases all that is required to complete the "booting" process is to wait for the computer to indicate a keyboard failure and then send an F1 code to the computer.
Accordingly, it is an object of this invention to provide circuitry for sending a data stream and a clock signal to a computer indicating that a keyboard is coupled thereto after it has gone into the aforementioned wait state and which is simpler in construction than the devices of prior art.